396 HISTORICAL GEOLOGY 



dacks and the Piedmont Plateau of eastern North America, Proter- 

 ozoic rocks have been identified with some certainty. 



Rocks of this age are believed to occur on other continents, but 

 their correlation has not yet been definitely determined. In China, 



Fig. 370. — A generalized section through the Black Hills, South Dakota, showing 

 the basal Archaeozoic rocks underlying the Cambrian and younger strata. 



for example, the Pre-Cambrian rocks have a threefold division, the 

 upper two of which are believed to be Proterozoic. 



Iron and Copper Deposits. — A discussion of the Proterozoic 

 would be incomplete without mention of the valuable deposits of 

 iron which they contain. In the five years previous to 1914, 

 216,981,280 long tons of iron ore were mined from the Proterozoic 

 rocks of the Lake Superior region alone, making this the most im- 

 portant iron-ore center in the world. The ore, chiefly as hematite 

 (Fe 2 3 ), occurs in the form of thick beds in the sedimentary strata. 

 Originally some of the formations contained large quantities of iron 

 minerals intermingled with silica and other non-metallic minerals, 

 and if it had remained in this state would probably not have been 

 of commercial value. The iron ore was later concentrated through 

 the agency of underground waters which dissolved out and carried 

 away the silica and other impurities, leaving pure, or nearly pure, 

 iron ore. Some deposits were further enriched by "replacement" 

 (p. 372), ore being deposited as the non-metallic minerals were 

 removed. 



One of the greatest known deposits of native copper occurs in the 

 rocks of the Keweenawan system of the Lake Superior region. The 

 copper occurs in the cracks of igneous rocks, in the pores of some of 

 the lava flows, and in the spaces between the pebbles and grains of 

 sand of rhc conglomerates and sandstones. The copper was originally 

 diffused in small quantities through the lava, but was partly dissolved 

 out by underground water, carried into porous layers, and there de- 

 posited, m sonic cases in such quantities as to constitute a cementing 

 material. 



Life of the Proterozoic Era. — The indirect evidences of life in the 

 Proterozoic are more abundant than in the Archaeozoic, although of 

 much the same character. Limestones imply but do not prove the 



